FIELD OF THE INVENTION
The invention relates to a gas-cooled turbogenerator having a pressure-tight casing with a stator disposed therein and a rotor. Feed and discharge devices for providing a cooling gas are also disposed in the casing along with cooling devices for the cooling gas. The stator and rotor have longitudinal slots for accommodating copper conductor bars surrounded by insulation. In the prior art, the volumetric ratio of the insulation to conductor bar volume is at least 1.0 for each longitudinal slot.
The term turbogenerator generally refers to synchronous generators, usually three-phase, two-pole, which are driven by a steam or gas turbine. In this case, it is immaterial whether they are intended for high or smaller outputs.
It has generally been known for a long time to dissipate the heat loss in generators with cooling gases. In this case, the cooling gas originally provided was air. Fans for moving the cooling gas are fastened for this purpose usually to the rotor shaft or acted upon by the rotor shaft. The cooling gas flows in at one end face or both end faces of the casing of the turbogenerator and flows through the casing and comes out again at the opposite end face or through corresponding outlet openings.
The great advantage of this type of construction has its roots in the simple construction, which requires no complicated casing for the generator but only an enclosure to deflect the air flow and in addition only a supporting frame for the stator.
In known generators, the enclosure is configured to be split along the longitudinal axis, so that the top part is removable and permits access to the stator. However, air-cooled generators are only intended for low outputs up to a maximum of 300 MVA, since the cooling effect attainable with air is inevitably limited and therefore the requisite cooling is not ensured with air at higher outputs.
For generators having a higher output, the cooling medium used is preferably hydrogen gas (H2), which, compared with air, permits a markedly higher cooling capacity, which can be increased by pressurizing the hydrogen gas and by carrying out the cooling under super atmospheric pressure. However, cooling with hydrogen gas requires an additional, not inconsiderable outlay, which has an effect on the costs both during the procurement and later during operation of the generator. A considerable outlay is necessary for the gas-tight and pressure-tight casing with shaft seals, the auxiliary equipment for drying, cleaning and pressure-regulating the cooling medium, the equipment for purging the casing for example during repairs, and for the auxiliary equipment for the degassing of the sealing medium of the shaft seal and its pressure regulation. This is to be explained below in detail.
The outlay already starts during the construction of the casing for the generator, which casing is not constructed merely as a thin-walled air-directing hood, as in the case of air-cooled generators, but is constructed to be pressure-tight. This is necessary because the casing has to permanently withstand a certain internal super atmospheric pressure relative to the ambient pressure and therefore has to undergo an internal-pressure test at a maximum pressure of 1,000,000 MPa in order to thus ensure its safety against bursting.
Provided in the interior space of the casing is a gas guide, which accordingly deflects the hydrogen-gas flow in such a way that the cooling-gas flow sweeps over the heat-emitting winding regions and the laminated body as uniformly as possible and absorbs heat in the process. Furthermore, the internal guide ensures that the hydrogen-gas flow, after absorbing the heat loss of the generator, reaches a cooler. The cooler is provided for the hydrogen-gas flow and preferably has a secondary feed of water and in which the hydrogen gas gives off the absorbed heat before it is fed again to the winding for a further cooling cycle.
A further problem associated with the generator having hydrogen gas cooling is the virtually unavoidable losses of hydrogen gas, mainly as a result of leakages at the leadthrough of the rotor shaft through the casing of the generator. In order to prevent these losses or at least keep them low, the shaft leadthroughs are each provided with a shaft seal that is configured as an oil-bath seal.
In this case, the sealing oil is constantly pressurized by a specially provided pressure-regulating device in order to balance the gas pressure of the hydrogen gas. Also required is a degassing device, which degasses the sealing oil at regular intervals or continuously.
In addition, a drying device for the hydrogen gas is provided, which ensures the dryness of the H2.
Finally, in the event of any necessary reconditioning or repair, if the pressure casing has to be opened, it is necessary to first of all remove the hydrogen gas from the casing without oxygen being added and forming an explosive oxyhydrogen gas. For this purpose, the generator casing is purged with nitrogen gas or carbon dioxide.
From all this, it is found that the technical advantage which favors the use of hydrogen gas as the cooling medium must be carefully considered, since the extra outlay resulting from this compared with the generators cooled only with air cannot be overlooked.
No measures of this type are required in the case of the air-cooled generators, so the outlay associated therewith is markedly lower. However, as already mentioned, the attainable cooling capacity and thus the attainable generator output are also markedly lower than in the high-output generators having hydrogen-gas cooling.
A considerable number of hydrogen-gas-cooled generators of an older type of construction are now due for reconditioning, the operating cost of which is comparatively high in relation to generators of a newer type of construction and should be reduced if possible.